This invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle or mask) to a sensitive substrate using a charged particle beam. Microlithography is a key technique used in the fabrication of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, and micromachines. More specifically, the invention pertains to configuring a charged-particle-beam (CPB) microlithography column so as to optimize a combination of certain optical parameters.
As is well known, the degree of integration and miniaturization of microelectronic devices continues to increase. Fabrication of microelectronic devices also continues to increase in complexity, with a concomitant need for increasingly greater accuracy and precision. As noted above, microlithography is a key technique used in fabrication of microelectronic devices. Microlithography methods that are mostly used today, so-called xe2x80x9copticalxe2x80x9d microlithography methods, are based upon use of light (especially deep UV light such as produced by an excimer laser) as a microlithographic energy beam. However, despite spectacular refinements in optical microlithography, the maximal resolution obtainable using optical microlithography is limited by the diffraction of light, and current optical microlithography systems operate at or near their theoretical resolution limits.
In the search for ever-greater resolution, several alternative microlithographic approaches have been investigated extensively. For example, considerable attention has been devoted to performing microlithography using an X-ray beam. However, X-ray microlithography currently is impractical due to several reasons including the great difficulty in making X-ray microlithography reticles.
Another approach that has received considerable attention is charged-particle-beam (CPB) microlithography in which pattern transfer is performed using a charged particle beam (e.g., electron beam or ion beam) instead of a beam of light or X-rays. A number of key developments have been made in this field of microlithography, including key developments in the CPB optical systems used such systems. Exemplary developments include MOL (Moving Objective Lens; see Goto et al., Optik 48: 255, 1977), VAL (Variable Axis Lens; see Pfeiffer and Langner, J. Vac. Sci. Technol. 19:1058, 1981), VAIL (Variable Axis Immersion Lens; see Sturans et al., J. Vac. Sci. Technol. B8:1682, 1990). However, despite these developments, and others, optimal performance of CPB microlithography systems has not yet been achieved.
Other theoretically possible approaches to the development of optimal CPB optical systems include one based upon multi-stage deflection theory (see Hosokawa, Optik 56:21, 1980). Yet another approach offering prospects of excellent imaging with low distortion or blur utilizes a simple two-stage projection-lens configuration with six deflectors, wherein each deflector is optimized for its intended use (e.g., optimized with respect to inner diameter, angle, excitation current, and position in the CPB column).
The imaging performance of a CPB optical system is affected not only by geometric aberrations and chromatic aberrations addressed by the conventional approaches noted above. Imaging performance also is affected by blur and distortion due to Coulomb interactions between individual charged particles of the beam (this phenomenon is referred to herein as the xe2x80x9cCoulomb effectxe2x80x9d).
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the invention is to provide charged-particle-beam (CPB) microlithography apparatus that exhibit satisfactory correction of geometric aberrations and the Coulomb effect and that exhibit low overall aberration and blur. Another object is to provide microelectronic-device manufacturing methods utilizing such CPB microlithography apparatus.
To such ends and according to a first aspect of the invention, CPB microlithography apparatus are provided that direct a shaped charged particle beam (e.g., electron beam) onto a reticle to illuminate a selected region on the reticle and that direct a patterned beam from the reticle to a substrate. A projection-lens system is situated between the reticle and the substrate. According to an exemplary embodiment, the projection-lens system is configured to direct the patterned beam from the reticle to the substrate at a demagnfication ratio of 1/M, wherein 0 less than M. Also, according to the embodiment, the projection-lens system has a column length (in mm) from the reticle to the substrate of 250xc3x97M0.63xc2x110%. This expression also is applicable if 0 less than M less than 4.
As the column length is increased, the geometric aberration is observed to be reduced, but the Coulomb effect is observed to increase, generally resulting in deteriorated optical characteristics of the projection-lens system. Conversely, as the column length is decreased, the Coulomb effect is observed to decrease, but the geometric aberration is observed to increase, again generally resulting in deteriorated optical characteristics of the projection-lens system. These observations suggest that there is an optimal column length, in terms of achieving better optical characteristics. The inventors also observed that the column length can be different for any of various projection-lens systems, but that no major divergence in performance occurred even when regarding optimal column length solely as a function of the demagnification ratio of the lens system. If the column length is within the range indicated above, both blur (resulting from a combined effect of geometric aberration and the Coulomb effect) and geometric distortion are excellently reduced (e.g., blur of 70 nm or less and geometric distortion of 4 nm or less).
Desirably, the projection-lens system is a symmetric magnetic doublet comprising a collimating lens and a projection lens. Each of the collimating lens and the projection lens desirably has associated therewith a respective set of at least three deflectors.
According to another aspect of the invention, methods are provided for manufacturing a microelectronic device, wherein each of such methods includes a wafer-processing step performed using a CPB microlithography apparatus as summarized above.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.